Among various factors influencing quality of a semiconductor or a FPD in a semiconductor process and a FPD process, it is an important factor to control the thickness of a thin layer, so that it is necessary to directly monitor the thickness of the thin layer during the processes. ‘A thin layer’ is a base layer, i.e. a layer which is formed on a surface of a substrate while having an extremely minute thickness, and the thickness of the thin layer is within several tens of Å˜several μm. In order to apply such a thin layer to a specific use, it is necessary to know the thickness, composition, and physical and optical characteristics thereof. Particularly, it has recently become a general tendency to form multi-super thin layers on a substrate so as to increase the degree of integration of a semiconductor device. In order to develop such a high integration semiconductor device, it is necessary to exactly control a property of a thin layer, including a thickness thereof which is a factor exerting a remarkable influence on its property. There are various methods for measuring the thickness of a thin layer used in a semiconductor process, an application process, etc. Among these methods, a mechanical method using a stylus, an optical method, etc. are the most typical methods. In an optical method, the thickness of a thin layer can be measured by using a white light interferometer.
FIG. 1 is a view illustrating an embodiment of a conventional method for measuring a thickness.
With reference to FIG. 1, a transparent thin layer, which is a subjecting layer for measurement of a thickness, is stacked on a base layer 10, and an air layer 30 is formed on the subjecting layer 120. A first surface 21 includes a boundary surface between the air layer 30 and the subjecting layer 20, and a second surface 11 includes a boundary surface between the subjecting layer 20 and the base layer 10. The subjecting layer 20 is slanted while having a thickness in a linear shape.
When an interference ray is irradiated toward one position 22 of the first surface 21, in which the thickness of the subjecting layer 20 is relatively thick, by using a typical white light interferometer, an interference signal 41 generated from the one position 22 of the first surface 21 and an interference signal 42 generated from one position 12 of the second surface are obtained. The interference signal 41 generated from the first surface and the interference signal 42 generated from the subjecting layer are fully spaced from each other so that they can be separated from each other. Therefore, the thickness of the subjecting layer 20 can be obtained at the position 22 by using a difference between maximized values of both interference signals 41 and 42.
However, at a position 23 where the subjecting layer 20 has a relatively thin thickness, it is impossible for the thickness of the subjecting layer 20 to be obtained through the above described method. Particularly, when an interference ray is irradiated toward another position 23 of the first surface 21, in which the thickness of the subjecting layer 20 is relatively thin, the interference signal generated from the position 23 of the first surface and the interference signal generated from a position 13 of the second surface are overlapped, thereby generating one interference signal 43. As such, through interference signals overlapped each other, which can be generated at a position having a thin thickness, each maximized value of the interference signals required for obtaining the thickness of the subjecting layer 20 can not be extracted. Therefore, there is a problem in that interference signals can not be used to a transparent subjecting layer 20 having a thin thickness.